First there are no thing called *Best Boffs*. You can have a boff with Bo1 and it is best for you. (So in my opinion it just depends on the view how you're looking at it.)
On the other hand Rare/Very Rare boffs have special skills which can't be trained by yourself. I.e. You fully spec into Starship Cannon Weapons and your able to train your boffs Rapid Fire 3 which is your personal ability (Only if your a TAC officer!), for example.
Rare/very rare boffs you gain through leveling and may have e.g. Beam Overload 3 or Transfer Shield Strength 3. Those can be found on the exchange as well but be aware they are very expensive.
If you want boffs that increase the effectivness of your power-levels (Weapon power / shield power etc.) you're able to buy "Efficient Saurians". Those carry a 5+ Efficient trait which increases slighlty your power levels in space. (But! They only affect your power levels as long as you keep them at 75 or below).
Just go to the exchange and type Saurian in the search option. Most of them are expensive as well ( 10 million Energy Credits upwards).
Apart from that Rare/very rare boffs have skills which can't be trained by yourself there is no real difference between a *blue/purple* officer. Just get a cheap one on the exchange by switching the search option from uncommon to rare etc.